1. Field of the Invention
This invention relates to the field of AC power controllers and in particular to a phase-triggered control operable to apply power to a load during a controllable portion of each power half-cycle.
2. Prior Art
Power controllers have been proposed wherein the point of zero crossing in an AC signal is sensed, and a timing means triggered at the zero crossing is employed to turn on a thyristor such as a triac, for applying the supply voltage to a load during only a certain phase increment of each half cycle. Examples of such controls are disclosed, for example, in U.S. Pat. No. 4,311,956-Tolmie Jr., and U.S. Pat. No. 3,593,112-Coats et al. The objective is the same in each case, namely to determine by preferably-variable means the precise point at which the triac will be turned on, applying current to the load at substantially the full supply voltage until the next zero crossing when, unless again triggered, the change in voltage polarity will turn the triac off. In this sense, "on" and "off" denote the conducting and non-conducting states of the triac, respectively.
The Tolmie reference employs two comparators, one for sensing the point of zero crossing on the supply voltage, and the other adjustably determining the phase delay until triggering of the triac. A series regulated power supply is provided to obtain a fixed DC voltage to power the comparators, i.e., high gain operational amplifiers. This accurate voltage supply is also used to provide an accurate voltage reference for detecting zero crossings in a full wave rectified signal derived from the power line.
The patent to Coats et al discloses a controller which can be operated by computer outputs and the like, using timing elements including unijunction and field-effect transistors. The objective is detecting zero crossings in the supply power, and timing the interval to a point at which a triac will be turned on. Coats' controller, like Tolmie's controller, is a complicated and expensive device which requires precise DC voltage regulation for its respective elements. In Coats, a series of zener diodes are provided for accurately regulating the supply voltage to the timing elements. A series regulator is used in Tolmie.
Precise DC voltage control is normally considered necessary for supplying integrated circuit timers, unijunction transistor timers and the like. Without good supply voltage regulation, accurate results are normally not expected. Supply voltage is typically regulated using a series power transistor with a zener diode reference, directly or indirectly controlling the conductance of the series transistor. In addition to the cost of these components, a good series regulator generates some overhead in terms of heat sinks and the like, which would be avoided if this were routinely possible.
Power control devices employing triacs are very useful for AC switching purposes, and are sometimes employed as so-called solid state relays. Inasmuch as the switching is normally accomplished at a zero crossing, inductive surges and the like are minimized. However, in a phase-delay switching apparatus as involved in the foregoing references and in this application, the voltage applied to the load by the triac rises quickly at the point of triggering, to substantially the full supply voltage level at the time of triggering. Where the switching occurs prior to the peak supply voltage level, the load experiences a short delay at zero voltage between half cycles, followed by a sudden rise to the voltage level at a predetermined, adjustable time. Unless the phase delay is long enough to pass the peak voltage, the result of the control is that the RMS power applied to the load is adjustable, while the peak voltage of the supply is not reduced in the power applied to the load. This renders the control especially applicable for adjusting lighting systems, for example with florescent bulbs, wherein it is desirable that the supply voltage reach the peak voltage, at least for a time during a half cycle. A power control for an incandescent lamp is disclosed in U.S. Pat. No. 4,549,116-Andrews. Thyristor controls with triacs or SCRs are also useful in connection with other types of devices, for example, electric motors. As SCR-triggered control is disclosed in U.S. Pat. No. 3,447,057-Brown et al. Similarly, thyristor controls can be used at frequencies or voltage levels other than that of the conventional utility or generation driven power lines. U.S. Pat. No. 4,623,824-Scolari et al, for example, discloses an SCR-switched power control for a DC/AC inverter and voltage multiplier.
The present invention minimizes the number of components and the required precision of the components. A comparator senses zero crossings by comparing a full wave rectified periodic signal with a low-pass filtered full wave rectified signal, the comparator also being powered by this filtered signal, notwithstanding the lack of series regulation. Timers having RC timing elements adjustably determine the phase delay and provide a pulse for triggering the triac during each half cycle. The timers are also powered by the filtered full wave rectified signal. Preferably, a precision potentiometer is employed in the RC network for the timer determining the phase delay. This potentiometer may be a multi-turn pot with a numeric indicator for precise control. The apparatus is especially applicable to florescent lighting control and can be packaged as a unit operable to control one or more circuits.